A known seat reclining apparatus for a vehicle, which adjusts a reclining angle of a seatback relative to a seat cushion, is disclosed in, for example, JP2002-177083A (hereinafter referred to as Patent reference 1). The known seat reclining apparatus for the vehicle disclosed in Patent reference 1 includes a gear plate provided with internal teeth, a base plate supported by the gear plate, pawls of which movement in a radial direction is guided by guide walls provided at the base plate and a cam engaging with the pawls. A rotation of the cam causes the pawls guided by the base plate (the guide walls) to reciprocate in the radial direction, thereby bringing the internal teeth of the gear plate and external teeth of each pawl into engagement with each other and releasing the engagement thereof. Thus, a rotation of the gear plate relative to the base plate is restricted and allowed. Accordingly, a rotation of the seatback relative to the seat cushion is restricted and allowed, and the seatback may be adjusted and retained at an inclination angle suitable for, for example, an occupant to be seated.
In addition, according to the known seat reclining apparatus for the vehicle disclosed in Patent reference 1, a technique for reducing an operating force for releasing the engagement between the internal teeth of the gear plate and the external teeth of each of the pawls in a case where, for example, the occupant is leaning against the seatback is disclosed. In order to facilitate a smooth reciprocating movement of each pawl in the radial direction, a distance between adjacent guide walls is usually set to be slightly greater than a width of each pawl, and thus a clearance is provided between the guide wall and the pawl. In this case, when the occupant is leaning against the seatback while the engagement between the internal teeth of the gear plate and the external teeth of each pawl is coming to be released, each pawl that has started moving in the radial direction may incline in a circumferential direction (that is, relative to the internal teeth) within a range of the clearance in a manner that one of the external teeth serves as a supporting point. As the pawl inclines, it presses the base plate (the guide wall), resulting in an increased frictional force of the pawl sliding on the base plate, thereby requiring a larger operating force.
Consequently, according to the known seat reclining apparatus for the vehicle disclosed in Patent reference 1, the external teeth of each pawl are formed so that a clearance is provided between a tooth face of at least one of the external teeth, which is positioned at each end in a width direction, and a tooth face of the corresponding internal tooth in a state where the internal teeth and the external teeth are engaged with each other. Thus, even though the occupant is leaning against the seat back while the engagement between the internal teeth of the gear plate and the external teeth of each pawl is coming to be released, a pressing force of each pawl against the base plate (the guide wall) is reduced, thereby reducing the frictional force of the pawl sliding on the base plate.
As each pawl includes plural external teeth, in a case where the pawl inclines in the circumferential direction in a manner described above, the engagement state between the external teeth of the pawl and the internal teeth of the gear plate may not be released uniformly or evenly. Part of the external teeth of the pawl may be caught by the internal teeth of the gear plate before a release of the engaged state is completed for the following reason. In a state where no external force is applied to a gear plate side, the cam pulls each pawl toward a radial center for unlocking. However, in a state where the external force is applied to the gear plate side, a component force toward a central direction is applied to each pawl depending on a pressure angle of a tooth contact area, and thus the pawl tends to disengage before the cam allows the pawl to do so in order to fill the clearance generated between a back portion of each pawl and a protruding portion of the cam. At this time, an abnormal noise is generated, thereby deteriorating an operating feeling.
A need thus exists for a seat reclining apparatus for a vehicle which is not susceptible to the drawback mentioned above.